In this paper the densification of a cylindrical pore due to a first order heterogeneous deposition reaction on the wall is described. The basic understanding of these mechanisms can aid in the prediction of the infiltration time and composite porosity after densification. The results of the modelling are compared with experimental results which are reported in an additional paper, i.e. Part 2 of this study. The main characteristic parameter that determines the infiltration behaviour is the gas velocity.